1. Field of the Invention
The present invention relates to improvements in or relating to a semiconductor photo-electrically-sensitive device which has at least a non-single-crystal semiconductor layer member having at least one intrinsic non-single-crystal semiconductor layer.
2. Description of the Prior Art
Heretofore there have been proposed a variety of semiconductor photo-electrically-sensitive devices of the type that have at least one non-single-crystal semiconductor layer member having at least one intrinsic non-single-crystal semiconductor layer.
In the semiconductor photo-electrically-sensitive device of such a structure, the intrinsic non-single-crystal semiconductor layer exhibits photo-conductivity corresponding to the intensity of incident light. Usually, the intrinsic non-single-crystal semiconductor layer contains hydrogen or a halogen as a neutralizer of recombination centers which would otherwise exist in large quantities since the intrinsic semiconductor layer is formed of a non-single-crystal semiconductor. This prevents the photo-sensitivity of the intrinsic non-single-crystal semiconductor layer from being lowered by recombination centers.
The photo-sensitivity of this kind of conventional semiconductor photo-electrically-sensitive device is very low and readily changes with intensity of incident light or temperature.
As a result of various experiments, the present inventor has found that one of the reasons for the low photo-sensitivity and its instability is that in the case where the intrinsic non-single-crystal semiconductor layer of the non-single-crystal semiconductor layer member is formed inevitably containing sodium as an impurity, the sodium content is as large as 10.sup.20 atoms/cm.sup.3 or more.
Moreover, the present inventor has found that such a large sodium content lowers the photo-sensitivity of the semiconductor photo-electrically-sensitive device and gives rise to the instability of the photo-sensitivity for the following reasons:
In a case where the intrinsic non-single-crystal semiconductor layer contains sodium in as high a concentration as 10.sup.20 atoms/cm.sup.3 or more, a large number of clusters of sodium are created in the intrinsic non-single-crystal semiconductor layer and these clusters of sodium serve as recombination centers of photo carriers. Accordingly, when the sodium content is large as mentioned above, the intrinsic non-single-crystal semiconductor layer contains a number of recombination centers of photo carriers which are not neutralized by a recombination center neutralizer. Consequently, photo carriers which are generated by the incidence of light in the intrinsic non-single-crystal semiconductor layer are recombined at the recombination centers, resulting in a heavy loss of the photo carriers. Further, the intrinsic non-single-crystal semiconductor layer, when containing sodium, creates dangling bonds of sodium, which serve as donor centers. In the case where the intrinsic non-single-crystal semiconductor layer contains sodium in as high a concentration as 10.sup.20 atoms/cm.sup.3 or more, it contains many dangling bonds of sodium acting as donor centers. In this case, the center level of the energy band in the widthwise direction thereof in the intrinsic non-single-crystal semiconductor layer relatively greatly deviates more toward the valence band than the Fermi level. Accordingly, the photosensitivity of the intrinsic non-single-crystal semiconductor layer depending upon the intensity of light is very low and changes with the intensity of the incident light or temperature. Further, the diffusion length of holes of the photo carriers in the intrinsic non-single-crystal semiconductor layer is short.
Moreover, the sodium contained in the intrinsic non-single-crystal semiconductor layer is combined with the material forming the layer. For instance, when the layer is formed of silicon, it has a combination expressed by the general formula Si--Na--Si. Accordingly, when the sodium content is as large as 10.sup.20 atoms/cm.sup.3 or more, the layer contains the combination of the material forming the layer and the sodium in large quantities.
The combination of the material forming the intrinsic non-single-crystal semiconductor layer and the sodium contained therein is decomposed by the incident light to create in the layer dangling bonds of the material forming it and dangling bonds of the sodium.
Therefore, in the case where the intrinsic non-single-crystal semiconductor layer contains sodium in as high a concentration as 10.sup.20 atoms/cm.sup.3 or more, the dangling bonds of the material forming the layer and the dangling bonds of sodium which are generated in the layer, will be greatly increased by the incident light. In such a case, the dangling bonds of the material forming the layer act as recombination centers of the photo carriers, and the loss of the photo carriers generated in the layer increases. As the dangling bonds of the sodium increase, the center level of the energy band in the widthwise direction thereof, which has greatly deviated more toward the valence band than the Fermi level, further deviates toward the valence band correspondingly, resulting in marked reduction of the photo carrier generating efficiency of the intrinsic non-single-crystal semiconductor layer. Also the diffusion length of holes of the photo carriers in the intrinsic non-single-crystal semiconductor layer is further reduced, markedly raising the dark conductivity of the layer.
In a state in which the photo carrier generating efficiency of the intrinsic non-single-crystal semiconductor layer has thus been lowered and the loss of the photo carriers in the layer and the dark conductivity of the layer have thus been increased, if the layer is heated, the dangling bonds of the material forming the layer and the dangling bonds of sodium, generated in large quantities in the layer, will be partly combined with each other to re-form the combination of the material forming the layer and the sodium. As a result, both the dangling bonds of the material forming the layer and the sodium content will be decreased. In the intrinsic non-single-crystal semiconductor layer, however, the dangling bonds of the material forming the layer and the dangling bonds of sodium still remain in large quantities. Consequently, the photo carrier generating efficiency of the intrinsic non-single-crystal semiconductor layer is very low and thus imposes a loss on the photo carriers in the layer, and the dark conductivity of the layer is extremely high. In addition, the photo carrier generating efficiency, the photoconductivity, the loss of photo carriers and the dark conductivity of the intrinsic non-single-crystal semiconductor layer, and accordingly the photo-sensitivity of the layer largely differ before and after heating.
The above is the reason found by the present inventor why the photo-sensitivity of the conventional semiconductor photo-electrically-sensitive device is low and readily varies with the intensity of incident light or temperature when the intrinsic non-single-crystal semiconductor contains sodium in as high a concentration as 10.sup.20 atoms/cm.sup.3 or more.
Further, the present inventor has also found that when the semiconductor contains oxygen in as high a concentration as 10.sup.20 atoms/cm.sup.3 or more, the photo-sensitivity of the conventional semiconductor photo-electrically-sensitive device is very low and fluctuates with the intensity of the incident light or temperature for the following reason:
When the intrinsic non-single-crystal semiconductor layer contains oxygen in as high a concentration as 10.sup.20 atoms/cm.sup.3 as referred to previously, the layer forms therein a number of clusters of oxygen. The clusters of oxygen act as combination centers of photo carriers as is the case with the clusters of sodium. Further, the intrinsic non-single-crystal semiconductor layer, when containing oxygen in such a high concentration as 10.sup.20 atoms/cm.sup.3.sub.1, the layer contains dangling bonds of oxygen acting as donor centers and the combination of the material forming the layer and oxygen in large quantities. The combination of the material forming the layer and oxygen is decomposed by the incident light to create in the layer dangling bonds of the material forming it and dangling bonds of the oxygen. If the layer is heated, the dangling bonds of the material forming the layer and the dangling bonds of oxygen will be slightly decreased small quantities but remain in the layer in large quantities.
The above is the reason found by the present inventor for why the photo-sensitivity of the conventional semiconductor photoelectric conversion device is very low and varies with the intensity of the incident light or temperature when the intrinsic non-single-crystal semiconductor layer contains oxygen in as high a concentration as 10.sup.20 atoms/cm.sup.3 or more.
Moreover, the present inventor has found that when the intrinsic non-single-crystal semiconductor layer contains sodium and oxygen each in as high a concentration as 10.sup.20 atoms/cm.sup.3 or more, the photo-sensitivity of the conventional semiconductor photo-electrically-sensitive device is very low and varies with the intensity of the incident light or temperature. The reason therefor will be apparent from the previous discussions.